The invention relates to a method of manufacturing a composite lightweight valve for internal combustion engines having a valve stem and a valve head joined to the valve stem.
EP 0 296 619 A1 likewise shows a multi-part lightweight valve for internal combustion engines, which has a valve head and a valve stem. At the top, the valve head is provided with a blind bore for receiving the head-side end of the stem, which can be secured in the blind bore by shrink-fitting, cold pressing, soldering, by a mechanical joint or by a combination of such joining techniques. In an embodiment illustrated in the drawing of that document, the interior of the blind bore is of undulating design, the end-side wall of the stem tube being widened under the influence of pressure and local heating. In this way, the stem is intended to be engaged in a positively locking manner by the undulations in the bore.
A drawback of the lightweight valve which is known from EP 0 296 619 A1 is that the joint between valve stem and valve head is not sufficiently able to withstand the static and dynamic loads, which are considerable both with regard to thermal loads and with regard to mechanical loads. Temperature fluctuations from xe2x88x9220xc2x0 C. to approximately 900xc2x0 C. may arise, sometimes occurring within a short time, while the high temperature may, under certain circumstances, be effective for an extended period. Secondly, the joint is also simultaneously exposed to high dynamic and static loads in the tensile direction. This set of loads may lead to the joint between the valve stem and the valve head becoming loose after a relatively short operating time of the internal combustion engine, which could result in failure and complete destruction of the internal combustion engine.
Therefore, the object of the invention is to improve a composite lightweight valve in such a manner that it is capable of withstanding the thermal and mechanical loads occurring during engine operation.
In a method of manufacturing a composite lightweight valve for internal combustion engines having a solid valve head provided with a center opening and a valve stem with a shoulder received in the center opening of the valve head for engaging the valve head in a positively locking manner, the valve components are permanently joined to one another by hot upsetting so as to be capable of withstanding the thermal and mechanical effective during engine operation.
A significant advantage of the invention is that, as a result of the cooling after the upsetting operation, considerable pre-stresses, which act in the direction of the longitudinal axis of the valve, i.e. axially, are produced in the head-side end of the valve stem, and these pre-stresses provide for firm engagement of the valve head with the stem shoulder and a strong connection between the head and the valve stem. This connection is even stronger than a weld connection. This means that the fatigue strength is higher than that of a welded valve, for example. Because of the positive locking connection between the valve stem and the valve head in both axial directions, namely compression and tension, the joint is able to withstand the thermal and mechanical loads, which arise when the engine is operating without damage to the valve for an extended period of time. Moreover, this joining technique reduces manufacturing outlay to only a few tasks, specifically, to the outlay relating to the insertion operation (inserting the stem end into the center bore), to the upsetting operation and if appropriate, to a grinding operation (head-side stem end).
The inventive joining technique for the composite lightweight valve allows freedom of choice in terms of the materials used, in particular with regard to the valve head. It may, for example, be made from a material, which cannot be welded, or specifically, a material, which cannot be welded to the material of the valve stem.